| Research Abstract |
Under the assumption that errors in DNA replication are the major source of mutations that contribute to molecular evolution, we showed that different numbers of germ-cell division between males and females result in different mutation frequencies between autosomes and sex chromosomes. The expected mutation frequency, R_<X/A>, of X chromosome and that, R_<Y/A>, of Y chromosome relative to the mutation frequency of autosomes were shown to be 2/3 and 2 in the XX female/XY male system, respectively, if male to female ratio of the number of germ-cell divisions (alpha) is very large. To test this, we compared nucleotide sequences of autosome- and X-linked genes between human and mouse(or rat) and analyzed the evolutionary rates of silent substitutions. The average rate of silent substitutions of X-linked genes relative to that of autosome-linked genes (R^'_) was shown to be 0.58, being close to the expected value 2/3. For the relative evolutionary rate R^'_ of a Y-linked gene, we observed R^'_=2.2, which again is close to the expected value 2. From the strong correlation between the observed R' and the expected R for alpha>>1, we proposed that males possibly serve as a mutation generator. We also found that, in X-linked genes, the rate of pseudogene evolution is 1.54 times as high as their functional counterpart and rodents evolve at 2.3 times the rate of non-rodent mammals. These properties agree well quantitatively with those found in autosomal genes, suggesting uniformly reduced mutation frequency over the whole X chromosomal regions. The relative mutation frequencies of the ZW female/ZZ male system were shown to be a reversal of those of XX/YY system: For alpha>>1, R_<Z/A>(=4/3)>1>>R_<W/A>. A very small value of R_<w/A> is expected. An analysis of avian genes would provide a clear-cut proof of this model.
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